In recent years, the Japanese diet has changed, and occurrence of lifestyle-related diseases (both symptoms and signs thereof) such as hypertension, diabetes, brain infarction, and heart diseases has increased not only in the elder generation but also in the younger generation, posing serious problems. In many cases, such lifestyle-related diseases are pointed out to be closely related to obesity. Thus, treatment and prevention of obesity through improvement of diet and physical exercise are important for preventing lifestyle-related diseases.
The most common way to control diet is to limit calorie intake. However, this is not always a preferred method, since unavoidable changes in quantity and quality of diet are a painful burden, possibly causing stress. If conversion of ingested food into energy through metabolism in the living body is inhibited or delayed, calorie intake can be restricted without controlling diet, which is a painful task.
Hitherto, a substance inhibiting the activity of α-amylase, which is a hydrolase for starch as an energy source, has been considered effective for dieting. Therefore, extensive studies have been conducted on such substances, and a study has revealed that an extract produced from guava leaves possesses excellent α-amylase inhibitory activity (see Patent Document 1).
Guava (Psidium guajava L.) is a plant originating in tropical America and belonging to Myrtaceae and is grown in, for example, tropical regions, Taiwan, and Okinawa. The juice thereof is a very popular beverage. Guava leaf extract contains large amounts of polyphenol substances, tannin substances, saponin, ellagic acid glycosides, flavonoids, etc., and a study revealed that the extract exhibits, in addition to the aforementioned α-amylase inhibitory activity, an effect of inhibiting formation of lipid peroxide (see Patent Document 2). There has been proposed use of guava leaf extract as an ingredient of foods and beverages.
However, when a guava leaf extract is stored for a long period of time, polyphenol substances and other ingredients make the extract cloudy, or form precipitates. Thus, when a beverage or the like containing such a guava leaf extract is charged into a container, the appearance of the beverage product is impaired, in some cases lowering motivation of consumers for purchasing the product.
In general, methods for maintaining quality of a plant extract containing large amounts of polyphenols and tannins have been reported. For example, filtration through diatomaceous earth (see Patent Document 3), removal of polyphenols with adsorbing resin (see Patent Document 4), and decomposition of tannins by use of tannase (see Patent Document 5) have been reported. Through performing these processes, some ingredients of guava leaf extract can be removed. However, since some of the removed ingredients are thought to be related to advantageous functions of guava, such conventionally employed treatments are not necessarily preferable. From another aspect, there has been proposed an approach for producing a guava leaf extract suitable for long-term storage, including incorporating stevia into the extract (see Patent Document 6). However, use of an additive should be avoided to the greatest extent.    Patent Document 1: JP-B-2670742    Patent Document 2: JP-A-1999-75770    Patent Document 3: JP-A-1992-311348    Patent Document 4: JP-A-1997-220053    Patent Document 5: JP-A-1994-269246    Patent Document 6: JP-A-1999-46737